In recent years, in order to enhance efficiency in semiconductor light-emitting elements such as an OLED, a fluorescent member and an LED, the improvement of the efficiency of extracting light from the semiconductor light-emitting element has been examined. The semiconductor light-emitting element described above has a structure in which a high refractive index region including a light-emitting portion therewithin is sandwiched between low refractive index regions. Hence, light emitted by the light-emitting portion of the semiconductor light-emitting element is brought into a waveguide mode in which the emitted light is wave-guided within the high refractive index region, is trapped within the high refractive index region, is absorbed in a waveguide process, is transformed into heat and is attenuated. As described above, in the semiconductor light-emitting element, it is impossible to extract the emitted light from the semiconductor light-emitting element, with the result that the light extraction efficiency is significantly and disadvantageously reduced.
In the case of an LED element, as will be described later, a light extraction efficiency LEE and an internal quantum efficiency IQE or a light extraction efficiency LEE and an electron injection efficiency EIE are simultaneously improved, and thus it is possible to manufacture a high efficiency LED element.
A GaN semiconductor element such as a blue LED is manufactured by depositing an n-type semiconductor layer, a light-emitting layer and a p-type semiconductor layer on a single crystal substrate by epitaxial growth. As the single crystal substrate, a sapphire single crystal substrate or a SiC single crystal substrate is generally used. However, since a lattice mismatch is present between a sapphire crystal and a GaN semiconductor crystal, dislocations are generated within the GaN semiconductor crystal (see, for example, non-patent document 1). The dislocation density thereof reaches 1×109 pieces/cm2. The dislocations cause the internal quantum efficiency of the LED, that is, the light emission efficiency of the semiconductor to be decreased, with the result that an external quantum efficiency is lowered.
The refractive index of a GaN semiconductor layer is higher than that of a sapphire substrate. Hence, light generated within the semiconductor light-emitting layer is prevented from being emitted from an interface between the sapphire substrate and the GaN semiconductor layer at an angle equal to or more than a critical angle. In other words, the light forms a waveguide mode, and is, in a waveguide process, transformed into heat and is attenuated. Hence, the light extraction efficiency is lowered, with the result that the external quantum efficiency is lowered. Moreover, when a SiC substrate having a significantly high refractive index as a single crystal substrate is used, since light is prevented from being emitted from an interface between the SiC substrate and an air layer at an angle equal to or more than a critical angle, as in the case where the sapphire substrate is used, the light forms a waveguide mode, and thus the light extraction efficiency LEE is lowered.
In other words, since a dislocation defect within the semiconductor crystal causes the internal quantum efficiency to be lowered, and the formation of the waveguide mode causes the light extraction efficiency to be lowered, the external quantum efficiency of the LED is significantly lowered.
Hence, a technology is proposed in which a concave-convex structure is provided in a single crystal substrate to change a light waveguide direction in a semiconductor crystal layer and thus a light extraction efficiency is increased (see, for example, patent document 1).
A technology is also proposed in which the size of a concave-convex structure provided in a single crystal substrate is on the order of nanometers, and thus the pattern of the concave-convex structure is brought into random arrangement (see, for example, patent document 2). It is reported that when the size of a pattern provided on a signal crystal substrate is on the order of nanometers, as compared with a pattern on the order of micrometers, the light emission efficiency of an LED is enhanced (see, for example, patent document 2).
Furthermore, a GaN semiconductor light-emitting element is proposed in which in order to enhance an electron injection efficiency EIE, a concave-convex structure is provided on the upper surface of a p-type semiconductor layer to reduce a contact resistance with a transparent conductive film (see patent document 3).